Police stop cars at a checkpoint near the U.S. embassy in Sanaa, Yemen. The State Department on Tuesday ordered non-essential personnel at the U.S. Embassy in Yemen to leave the country. (ASSOCIATED PRESS / Hani Mohammed)

WASHINGTON (AP) — The map of closed American embassies — and those that remain open — in the Middle East and Africa provides a window into the Obama administration’s concern about a potentially imminent al-Qaida terrorist attack on overseas U.S. interests.

While diplomatic missions across a broad swath of the Arab world are affected, some, including in capitals that have been targets for extremists in the past, are not. And those chosen for closure in Africa and the Indian Ocean suggest that the fear may be as much about the vulnerability of certain embassies and staff and the range of increasingly mobile terrorists as it is about specific threats.

One apparently key factor: How significant is the security that is now in place?

A total of 19 U.S. embassies and consulates in 16 countries have been ordered to close to the public until Saturday. They run along a jagged, east-to-south crescent from Libya through the Persian Gulf to Rwanda and include the island nations of Madagascar and Mauritius. That’s fewer missions in fewer nations than were ordered closed this past Sunday in the administration’s initial response to intelligence that al-Qaida in the Arabian Peninsula was gearing up for an attack.

The changes, coupled with the inclusions and omissions, show how the threat analysis has evolved.

President Barack Obama, in his first public comments on the matter, said the terror threat is “significant enough that we’re taking every precaution.” Despite the global travel warning, he said Americans should not cancel their overseas travel plans but should “show some common sense.”

“There are some countries where you’re less likely to experience a terrorist attack, there are some that are more dangerous,” Obama said in an interview with NBC’s “The Tonight Show” taped for broadcast Tuesday night.

According to the State Department, the closures are all the result of the same intelligence on the threat. Yet, that intelligence stream appears to be significantly diffuse, covering embassies and other posts stretching 4,800 miles from Tripoli, Libya, to Port Louis, Mauritius, and not limited to Muslim or Muslim-majority nations.

“It is the same stream that we’ve referenced in travel warnings since Sunday,” department spokeswoman Jen Psaki said Tuesday, adding: “Obviously, there’s new information.” She would not elaborate.

Clearly, Yemen, where the department ordered most U.S. government employees to leave early Tuesday, is at the center. The base of Al-Qaida in the Arabian Peninsula, Yemen has been the subject of a dire and continuous U.S. travel warning since 2002.

An intercepted secret message between al-Qaida chief Ayman al-Zawahri and his deputy in Yemen about plans for a major terror attack triggered the closures. Al-Zawahri’s message to Nasser al-Wahishi was picked up several weeks ago and appeared to initially target Yemen, according to officials familiar with the matter. The threat was expanded to include American or other Western sites abroad, officials said, indicating the target could be a single embassy, a number of posts or some other site.

Based on their very close proximity to Yemen, U.S. facilities in that country’s closest neighbors — Oman, Qatar, Bahrain, the United Arab Emirates, Saudi Arabia and Djibouti — could be considered logical targets, as could posts in Egypt, Jordan, Libya and Sudan. But Lebanon, which has been the site of major anti-U.S. terrorist attacks in the past, is not included. And, neither are Morocco and Tunisia, where extremists have also struck previously.

Missions in Mauritania, Algeria, Iraq, Afghanistan and Bangladesh, where attacks have long been frequent, were covered in the initial closure order but were allowed to reopen on Monday.

U.S. diplomatic posts in Pakistan, which have been hit repeatedly by notoriously active extremists, were never closed in response to the latest threat. Nor were those in Indonesia or Kenya or Tanzania, all of which have come under attack by al-Qaida or its affiliates. Thursday will mark the 15th anniversary of the 1998 attacks on the U.S. embassies in Nairobi and Dar es Salaam.

In each of those countries, past terrorism experience has led the U.S. to significantly increase security at its missions.

Thus, it is significant that the State Department deemed it prudent to close its embassies in Rwanda and Burundi, two tiny central African countries known for genocide and brutal tribal clashes. It also shuttered embassies in Madagascar and Mauritius. Apart from domestic political instability that has led to violence, Madagascar has not been a particular security concern for the United States in the past. Likewise, Mauritius, a sleepy but stable and relatively prosperous country without an army, has not been on the terrorism radar before.

Yet, uneven border policing and inconsistent performance by security forces in each of those four countries, coupled with a less intense day-to-day security focus from Washington could make them attractive to would-be attackers.